Our planet is blue inside and out. A massive reservoir of water has been discovered deep beneath a volcano in the Andes, and Earth’s interior may be dotted with similar wet pockets lurking below other major volcanoes.

The unexpected water, which is mixed with partially melted magma, could help to explain why and how eruptions happen.

This water may also be playing a role in the formation of the continental crust we live on, and could be further evidence that our planet has had water circulating in its interior since its formation.

Deep Earth in a lab

Jon Blundy of the University of Bristol, UK, and his colleagues made the discovery while studying a huge “anomaly” 15 kilometres beneath the currently dormant Uturuncu volcano in the Bolivian Andes. The anomaly, called the Altiplano-Puna magma body, slows down seismic waves and conducts electricity, unlike surrounding magma.

Blundy’s team took rocks that were spat out by an eruption of Uturuncu 500,000 years ago and mixed them with varying amounts of water before exposing them in the lab to conditions mimicking those in the anomaly.

This included pressures 30,000 times as high as atmospheric pressure, and temperatures up to 1500 °C. “We reproduced conditions deep in the Earth in the lab,” says Blundy.

They found that at a particular water content, the electrical conductivity exactly matched the value measured in the anomaly. “By weight, we calculated it contains 8 to 10 per cent water,” says Blundy.

Staggering amount

The Altiplano-Puna magma body is known to be around half a million cubic kilometres in volume, so the researchers estimate it must contain a similar amount of water to some of the largest freshwater lakes on Earth. “It’s probably somewhere between Lake Superior and Lake Huron,” says Blundy. “It’s a staggeringly large amount.”

Other anomalies with similar unexplained conductivity have been discovered beneath other volcanoes, such as those in the Taupo Volcanic Zone in New Zealand, and Mount St Helen’s in Washington State, which erupted spectacularly in 1980. It’s likely that these are also signs of secret reservoirs.

“This study illuminates a new feature of Earth’s deep-water cycle, and reminds us how little we know about the pathway of water through Earth’s crust and mantle systems on geologic timescales,” says Steve Jacobsen of Northwestern University in Evanston, Illinois, whose team previously discovered a reservoir of water three times the volume of all the oceans 700 kilometres down in the mantle.

Such discoveries add to growing evidence that significant amounts of water exist in Earth’s interior, some of which may even have been the source of today’s oceans. It could be that the water that makes our planet habitable was present in the dust that coalesced to create Earth, rather than arriving later on ice-rich comets or asteroids.

Beyond our reach

We can forget about extracting the newly found water. “It’s dissolved in partially melted rock at 950 to 1000 °C, so it’s not accessible,” says Blundy.

But increased water content in magma may help to explain the composition of continental crust rocks. When magma in the mantle – mainly composed of basalt – rises up into the crust, the water helps to enrich the basalt with silica and deplete it of magnesium, eventually forming rocks like the andesite found beneath the Andes.

“The process in Uturuncu is a microcosm of continental crust formation, and involves much more water than we thought, probably twice as much,” says Blundy.

Water is also one of the volatile components dissolved in magma that drive volcanic eruptions, he says. “Dissolved at shallower depths where the pressure is lower, it comes out as bubbles, which end up as an explosive eruption.”

In the future, understanding more about how water can trigger eruptions could help volcanologists better interpret seismic activity, perhaps improving predictions. “Our results will hopefully improve our ability to interpret these signals of unrest,” says Blundy.